Pregnancy paranoid

I had sex the middle of April (used birth control and a condom) and got my period 1.5 weeks after and just finished another regular cycle. 
I took 3 pregnancy tests this past week (over a month since I last had sex) and they were negative. Ive been having a lot of stomach issues but my doctor thinks it could be lactose intolerance. I'm still so paranoid I'm pregnant. Can someone help?